1. Field of the Invention
The present invention relates to a green sensitized photographic silver halide sensitive materials which are exposed to radiation by contact with an intensifying screen comprising a fluorescent substance which emits light in the green spectral range.
2. Description of the Prior Art
An intensifying paper or a fluorescent plate together with a conventional silver halide photographic film is often used in carrying out X-ray recording in order to enhance the recording sensitivity to X-rays. Enhancing the recording sensitivity to X-rays to obtain effective radiographic images with less doses of X-rays in highly desirable since excess exposure to X-rays is harmful to the human body. Recording fluorescent images on a silver halide photosensitive material using the above procedure is one of the most effective methods to accomplish this purpose.
As fluorescent substances hitherto used for such purposes, there are those which emit blue fluorescent light, such as barium sulfate activated with strontium, barium sulfate activated with lead, barium sulfate activated with silver, calcium tungstate (CaWO.sub.4) activated with lead, zinc sulfide activated with silver and barium phosphate (Ba.sub.3 (PO.sub.4).sub.2) activated by europium, etc. For purposes of medical treatment, a fluorescent intensifying screen used together with a silver halide radiographic material contains, in fact, calcium tungstate or barium sulfate activated with lead as a fluorescent substance. Recently, studies on intensifying fluorescent screens having a high emission energy intensity have been performed because of increases of requirements on X-ray photography for medical treatment. Particularly, it is known from reports of the Lockheed Aircraft Corp. that rare earth oxysulfide and oxyhalide fluorescent substances activated by another rare earth element have a high emission energy intensity.
Phosphorescent or fluorescent substances used for the fluorescent screen which emits visible light used in the present invention preferably include elements of an atomic number of 39 or from 57 to 71 (for example, yttrium, gadolinium, lanthanum or cerium, etc.). Particularly preferred substances are rare earth oxysulfide and oxyhalide fluoroscent substances activated by another rare earth element, such as lanthanum or gadolinium oxybromide or oxychloride activated by terbium or dysprosium, or lanthanum or gadolinium oxysulfide activated by terbium, europium or a mixture of terbium and samarium, etc. Such rare earth fluorescent substances are described in German Pat. No. 1,282,819, French Pat. Nos. 1,540,341, 1,580,544 and 2,021,397, French Patent of Addition No. 94,579 (Patent of Addition of Basic French Pat. No. 1,473,531), U.S. Pat. Nos. 3,546,128, 3,418,246 and 3,418,247, British Pat. No. 1,414,456, "Rare Earth Oxysulfide X-ray Phosphors" reported by K. A. Wickersheim in IEEE Nuclear Science Symposium in San Francisco on Oct. 29-31, 1969, and "IEEE Transactions on Nuclear Science" pages 81-83, February 1972, written by R. A. Buchanan. Such rare earth fluorescent substances, and particularly gadolinium and lanthanum oxysulfide and oxyhalide activated by erbium, terbium or dysprosium, have a high X-ray stopping power or average absorption capacity and a high emission density and enable radiologists to use substantially lower X-ray dosage levels.
Specific fluorescent substances which are very suitable for use in the intensifying screens employed in the present invention, and which we cite merely as examples, are fluorescent substances having the following general formula: EQU M(p-q)M'.sub.q O.sub.p X
wherein M is at least one of the metals yttrium, lanthanum, gadolinium or lutetium, M' is at least one of the metals dysprosium, erbium, europium, holmium, neodymium, praseodymium, samarium, terbium, thulium or ytterbium, X is sulphur or halogen, q is 0.0002 to 0.02 and p is 1 when X is halogen or is 2 when X is sulphur.
It is economically advantageous that the amount of silver halide per unit area of the photosensitive material be small to shorten the period of time for processing, particularly for fixing. However, if a photosensitive material having a low silver halide content is used, the maximum density and contrast of the resultant images become comparatively low. Although an emulsion composed of fine particles is effective for obtaining a high maximum density, it is necessary to use high exposure amounts, because photographic sensitivity is low. Accordingly, it is preferred to use an emulsion which is composed of high sensitivity silver halide particles, whereby sufficiently high maximum density and high contrast can be obtained even if the amount of silver halide per unit area is low.
According to the present invention, preferred results are obtained even if a photographic material containing silver in an amount below about 8.6 g, preferably in an amount of from about 2 to below 8.6 g, per square meter is used. When a silver halide photographic material having silver halide layers on both faces of the support through which fluorescent rays pass is brought into contact with a fluorescent intensifying screen during X-ray exposure, a so-called "cross-over" phenomenon occurs. Light emitted from such a fluorescent screen not only forms black images on the silver halide emulsion layer adjacent the fluorescent screen but also forms blurred images on the silver halide emulsion layer placed on the reverse side of the support because a considerable amount of light passes through the support having a relatively high thickness. This phenomenon is called "cross-over". The degree of cross-over substantially controls the quality of the images formed in the photographic material.
In conventional radiographic recording materials containing a relatively large amount of silver halide per unit area, cross-over occurs to a lesser degree. However, silver halide photosensitive materials having a low silver halide content per unit area are highly affected by cross-over.
The method is known, for example, from Japanese Patent Application (OPI) No. 63,424/74 which comprises incorporating dyes which absorb light having the same wavelength range as that emitted by a fluorescent intensifying screen in at least one layer of a silver halide photographic material in order to reduce cross-over. The dyes used for such purpose are those used usually for inhibiting irradiation (scattering of light in emulsion layers), which usually cause desensitization when present in a silver halide emulsion layer. In fact, some degree of desensitization is inevitable because of the light absorption effect even if dyes which do not chemically cause desensitization are used. On the other hand, these dyes may be added to hydrophilic colloid layers other than a silver halide emulsion layer. However, they inevitably bring about some degree of desensitization because they migrate into the silver halide emulsion layers by diffusion as they are water soluble.
On the other hand, it is possible to dye the base support itself in order to reduce cross-over. However, a sufficient reduction of cross-over is not obtained by dyeing only the base support.
Further, it is preferred that X-ray sensitive materials (direct X-ray sensitive materials and indirect X-ray sensitive materials) be easily processed, for example, at development or fixing, particularly, in a light room. Such X-ray photographic materials have been processed under a safe light using, for example, a No. 7 safe light filter produced by Fuji Photo Film Co., Ltd. The spectral transmission curve of a No. 7 safe light filter is shown in FIG. 1. It is preferred that radiographic materials prepared using silver halide photographic emulsions have a high sensitivity to light emitted by the excitation of the above described improved green fluorescent substances and have a low sensitivity to safe light.
U.S. Pat. No. 3,953,215 Hinata et al is directed to silver halide materials supersensitized to the green region of the spectrum using a combination of cyanine dyes. Supersensitizing dye combinations for the green region of the spectrum are set forth in U.S. Pat. Nos. 3,832,189 and 3,854,955 to Shiba et al or in U.S. Pat. No. 3,864,134 to Ueda et al. U.S. Pat. Nos. 3,912,933 and 3,923,515 to Van Stappen show the use of a pair of intensifying screens in radiographic methods where silver halide is employed.